The present invention relates to the field of construction ceramics and more particularly to a dense silicon nitride composite material based on polycrystalline silicon nitride sintered bodies, as well as a method for manufacturing such a material. The present invention may be used as high-temperature components in building armatures and motors, as well as in the energy field.
Ceramic materials are far superior to metallic superalloys for usage at higher temperatures. Silicon nitride materials are especially suited for use at high temperatures, and in fact have excellent mechanical properties both at room temperature and at high temperatures.
However, one of the most significant disadvantages of these materials is their insufficient chemical resistance to oxygen at high temperatures. Usually, in order to alleviate this problem, the grain boundary phase in the material is modified (composition or crystallization of the amorphous glass phase) or various materials are combined in order to achieve an increase in oxidation resistance (minimization of the relative weight increase). However, even very small oxidation rates can cause serious damage by cracking and pore formation on the surface of the material that is stressed by stretching, which can in turn lead to rapid failure of the part. Therefore, the service life of such a part is often significantly reduced because of such damage, so that such parts are not suitable for applications which require a long service life (see J. Am. Ceram. Soc. 76 (11) 2919-2922 (1993)).
Tests have been performed in order to limit this damage and to increase the strength of the material after high-temperature treatment of up to 1200.degree. C. (see Komatsu et al., U.S. Pat. No. 4,407,971 counterpart of EP Patent No. 73,523, the entire disclosure of which is herein incorporated by reference) Suicides in the form of Mg.sub.2 Si, CaSi.sub.2, VSi.sub.2, CrSi.sub.2, MnSi, ZrSi.sub.2, NbSi.sub.2, MoSi.sub.2, TaSi.sub.2, and WSi.sub.2 were added in amounts of from 0.1 to 5 wt. % to a silicon nitride material. The results showed improved strength of these materials as compared to materials of the prior art. Nevertheless, the results obtained for parts made of these materials are still inadequate for use at higher temperatures and for a long service life.
Therefore, a goal of the present invention is to provide a dense silicon nitride composite material which has a long service life and high reliability even at higher temperatures, and a method for manufacturing such a material.
The dense silicon nitride composite material according to the present invention contains 3 to 50 wt. % of a reinforcing component, wherein the reinforcing component consists of a mixture of metal silicide phases with 10 to 90 wt. % Me.sub.5 Si.sub.3 and 90 to 10 wt. % either MeSi.sub.2 or MeSi.sub.2 and suicides of other stoichiometries, whereby once again, 10 to 90 wt. % is MeSi.sub.2 and 90 to 10 wt. % are suicides of other stoichiometries. In any case, Me is a metal or a mixture of metals. This means that the same metal or a different one may be involved in MeSi.sub.2 and Me.sub.5 Si.sub.3. In addition, one of the two or both of the Me's can be a mixture of metals.
In a preferred embodiment, the reinforcing component is present in amounts of from 5 to 40 wt. %. In another preferred embodiment, Me.sub.5 Si.sub.3 is present in amounts of from 3 to 10 wt. % in relation to the composite material. In another preferred embodiment, MeSi.sub.2 is present in amounts of from 8 to 20 wt. %, in relation to the composite material. In another preferred embodiment, MeSi.sub.2 and silicides with other stoichiometries are present in amounts of 10 to 30 wt. % in relation to the composite material.
Additionally, in a preferred embodiment, the metal is molybdenum. In another preferred embodiment, the metal is a metal or a mixture of metals selected from the group consisting of molybdenum, tungsten, chromium, tantalum, niobium, manganese, and vanadium.
It is also advantageous for the material according to the present invention to contain one or more sintering aids. Furthermore, it is also advantageous for the material according to the present invention to contain additional reinforcing components in the form of SiC, TiN, TiC or BN.
The present invention also discloses a method for making a dense silicon nitride composite material with 3 to 50 wt. % of a reinforcing component containing a mixture of metal suicides with 10 to 90 wt. % Me.sub.5 Si.sub.3 and 90 to 10 wt. % MeSi.sub.2 or MeSi.sub.2 and silicides with other stoichiometries, whereby if MeSi.sub.2 and suicides with other stoichiometries are used, once again, 10 to 90 wt. % is MeSi.sub.2 and 90 to 10 wt. % are silicides with other stoichiometries. In any case, Me is a metal or a mixture of metals. According to the present invention, the dense silicon nitride composite material is produced by sintering and/or hot pressing and/or hot isostatic pressing, and the reinforcing component is added in the form of metal silicide powder in powder metallurgy fashion or as a preliminary stage of the metal suicides whereby a nitrogen pressure/temperature ratio is set, as shown in the FIGURE, until closed porosity is reached, which results in the formation and stabilization of Me.sub.5 Si.sub.3.
According to a preferred embodiment, Me.sub.5 Si.sub.3 and MeSi.sub.2 are formed from one of their preliminary stages during plasma or gas phase synthesis of silicon nitride powder with metal dopings. In another preferred embodiment, Me.sub.5 Si.sub.3 and MeSi.sub.2 are formed from one of their preliminary stages through chemical synthesis with the decomposition of silicoorganic precursors with metal ions. In another preferred embodiment, Me.sub.5 Si.sub.3 and MeSi.sub.2 are formed from one of their preliminary stages by reduction of precipitated oxidic compounds of the metals. In yet another preferred embodiment, Me.sub.5 Si.sub.3 and MeSi.sub.2 are formed by powder metallurgy from metals, metal carbides, metal borides or metal nitrides.
Another advantage of the silicon nitride composite materials produced according to the present invention is improved creep properties under certain conditions.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.